1. Field of the Invention
This invention relates to an apparatus for the catalytic cracking of hydrocarbons. The apparatus comprises two independent transfer line reactors, each of which is associated with an independent cyclone separation system and wherein the cyclone separation systems are located within a common separator vessel. More particularly, the invention relates to an improved apparatus which can be used for the simultaneous fluidized catalytic cracking of a hydrocarbon feedstock and the recracking of a selected fraction of the products from said cracking without commingling the feedstock and the selected fraction to be recracked.
2. Description of the Prior Art
Catalytic cracking of heavy petroleum fractions is one of the major refining operations employed in the conversion of crude petroleum oils to useful products such as the fuels which are utilized by internal combustion engines. In fluidized catalytic cracking processes, high molecular weight hydrocarbon liquids and vapors are contacted with hot, finely divided, solid catalyst particles, either in a fluidized bed reactor or in an elongated transfer line or riser reactor, and maintained at an elevated temperature in a fluidized or dispersed state for a period of time sufficient to effect the desired degree of cracking to lower molecular weight hydrocarbons of the kind typically present in motor gasoline and distillate fuels.
Products from the catalytic cracking of heavy petroleum fractions have been of suitable quality for direct use as gasoline blending components. However, the recent trend toward lower sulfur and lead levels, in gasoline has made some form of reprocessing of the catalytic cracking product desirable. In addition, requirements for additional sources of light olefins for either chemical feedstock or alkylate feedstock and of light aromatics for chemical feedstock have also made reprocessing desirable.
A typical product mix obtained from the fluid catalytic cracking of a gas oil or heavier feedstock is given in the following Table:
______________________________________ WEIGHT PERCENT APPROXIMATE BASED ON BOILING PRODUCTS FRESH FEED RANGE .degree.C. ______________________________________ Light Naphtha 25 25 to 115 Heavy Naphtha 25 115 to 200 Light Cycle Oil 15 200 to 340 Butanes-Butylenes 10 -12 to +4 Propane-Propylene 5 -48 to -42 Ethane and Lighter 5 Heavy Cycle Oil 5 340 to 425 Decanted Oil 5 425 + Coke 5 Nonvolatile ______________________________________
It has been suggested to recycle the heavy cycle oil and decanted oil separated by fractionation by reincorporating them with the fresh feed, and it has also been proposed that benefits can be derived by recracking a selected boiling range intermediate fraction of the light cycle oil and the heavy naphtha. If selected boiling range fraction of the light cycle oil and heavy naphtha are reintroduced with the fresh feed, conversion is limited for the fraction is simply being subjected a second time to the same conditions. Accordingly, it is preferred to fractionate the cracked products and to recrack the selected boiling range fraction of either or both the heavy naphtha and light cycle oil, separately from the fresh feed and at an appropriate degree of severity.
U.S. Pat. No. 3,928,172 to Davis et al. discloses a method for improving the quality and volatility of a refinery gasoline pool which comprises catalytically cracking a gas oil, fractionating the resulting products, and recracking the heavy naphtha which is obtained from the fractionation. This patent discloses a catalytic cracking apparatus which comprises two independent vertical transfer line reactors wherein one transfer line reactor is for gas oil cracking and the other is for heavy naphtha recracking. Each transfer line reactor is directly connected to a separate cyclone separation system, and the hydrocarbon products flow from each cyclone system to a separate product recovery system. Davis et al. disclose a number of different apparatus configurations but do not disclose apparatus wherein a compact arrangement with separate riser reactors for feedstock and a cracked fraction open into the same separator vessel wherein provision to prevent commingling of the cracked products is provided.
U.S. Pat. No. 4,067,798 to Hauschildt et al. also discloses a method for improving the quality of a refinery gasoline pool. Hauschildt et al. teach that a feedstock comprising from 80 to 100 volume percent of a hydrocarbon fraction containing at least 25 weight percent of mono-aromatic hydrocarbons and boiling in the boiling range of heavy gasoline in combination with from 0 to 20 volume percent of a vaporizable hydrocarbon fraction can be catalytically cracked to produce a product having an increased low molecular weight olefin and mono-aromatic content, a substantially reduced sulfur content, and a high octane rating relative to the feedstock. It is disclosed that the fraction comprising from 80 to 100 volume percent of the feedstock is preferably a catalytic cracking product. It is also disclosed that the feedstock must be subjected to catalytic cracking in the absence of any other feedstock and the products must be isolated separately from any other products. Although the Hauschildt et al. process can be carried out using conventional apparatus for the catalytic cracking of hydrocarbons, there is a need for an apparatus which will permit the more efficient practice of this process.